Prior art attempts to mount the membrane keyboard sandwich structure onto a rigid support plate involved an extensive, substantially complete adhesive layer applied between the bottom surface of the membrane sandwich and the plate.
The complete or substantially complete adhesive layer used in the prior art to attach the membrane keyboard sandwich to the rigid support plate provides less than desirable results inasmuch as air bubbles may be trapped within the large adhesive region and effectively force the bottom layer of the sandwich away from the backing plate.
This results in a potential failure of the keyboard if the bubble is of sufficient size and location to force the bottom layer contact out of the plane of the support plate and thereby either cause an undesired contact between the contacts of that switch location or reduce the amount of distance between the two membrane mounted switch contacts thereby raising the possibility of unintentional contacts being made on an intermittent basis.
The problem of assembling the membrane keyboard sandwich onto the rigid support plate is further complicated when the sandwich is attached to the concave surface of a curved rigid support plate since forces may be transmitted to a key switch position through the membrane keyboard sandwich. This problem is caused by the inability to contact the entire adhesive layer to the support plate simultaneously when being assembled.
The venting of the individual cavities positioned between the electrical contacts of the membrane switch structure involved the forming of channels leading away from the cavities and passing into passages formed in the adhesive layer intermediate one membrane and the spacer layer and passing out to the atmosphere. U.S. Pat. No. 4,249,044 to Larson discloses such a series of channels for equalizing pressure interiorly of the switch with the atmosphere. Larson also discloses a chamber of air formed under the switch contact at the key switch position, which separates the bottom layer from its support member.
Larson further discloses the membrane switch structure attached to a support plate by an adhesive pattern which adheres the switch structure substantially completely around the periphery of the switch structure. Also adhesive, in a layer, is placed between the plate and the bottom layer of the switch structure but leaves regions of no adhesive under the switch site. This lack of adhesive attachment under the switch site leads to errors in that the bottom layer and its electrical contact may be displaced from the design location and not make and break as designed. Air pressure increase in the chamber beneath the switch position causes the upward movement of the electrical contact toward the electrical contact on the top layer of the switch structure, reducing the electrical contact separation or closing the switch.